The present invention relates generally to the field of catalysis for the reduction of emissions from internal combustion engines. More particularly, the present invention relates to a method and apparatus for heating a catalyst by spontaneous combustion of hydrogen introduced into the catalyst. More particularly still, the present invention relates to the conditioning through preheating of a standard three-way or two-way catalytic monolith in a vehicle powered by an internal combustion engine, such as an automobile.
The control and suppression of unwanted emissions created by the operation of an internal combustion engine is a primary consideration for engine designers and vehicle manufacturers because of nearly world-wide governmental requirements regarding acceptable emission levels. Over eighty percent (80%) of the unacceptable emissions or pollutants created by internal combustion engines equipped with catalytic converters occur during cold start operations. These pollutants are emitted for a period of one to three minutes after cold engine starting, in large part because that is the time period required for the catalyst to reach an efficient operating temperature. Therefore, even though the engine exhaust is flowing through the catalytic converter, until the exhaust heats the catalytic converter to its operating range from engine start up, the exhaust flow is only slightly catalyzed during that time period.
In order to meet governmental emission standards for internal combustion engine exhaust, a catalytic converter is located in the exhaust stream of the engine. The converter typically includes a canister holding a suitable catalyst, such as a three-way catalytic converter (TWC) catalyst monolith, that will oxygenate unburned, unacceptable components in the exhaust stream including hydrocarbons ("HC"), their partially oxidized derivatives such as aldehydes and carbon monoxide ("CO"), and at the same time reducing nitrogen oxides ("NO.sub.x "), after almost stoichiometric fuel burn with oxygen in the cylinders of the engine. The exhaust gas is passed through the catalyst monolith, thereby completing the oxygenation of unburned HC and CO, and the reduction of NO.sub.x in the exhaust to convert these unacceptable emissions into acceptable emissions. Certain unacceptable emissions in the exhaust stream, including unburned hydrocarbons and carbon monoxide, require an oxidation reaction to destroy them so that they end up as the corresponding oxides, e.g. water and carbon dioxide. On the other hand, NO.sub.x requires a reduction reaction to develop N.sub.2 and O.sub.2. In fact, the O.sub.2 product of this reduction contributes to the oxidation of the HC and CO in the exhaust.
TWC catalysts are currently formulated and designed to be effective over a specific operating range of both lean and rich fuel/air conditions and a specific operating temperature range. These particulate catalyst compositions enable optimization of the conversion of HC, CO, and NO.sub.x. This purification of the exhaust stream by the catalytic converter is dependent on the temperature of the exhaust gas and the catalytic converter works optimally at an elevated temperature, generally at or above 300.degree. C. The time span between when the exhaust emissions begin (i.e., "cold start"), until the time when the substrate heats up sufficiently for the catalyst to work efficiently, is generally referred to as the light-off time. Light-off temperature is generally defined as the temperature at which fifty percent (50%) of the emissions from the engine are being converted as they pass through the catalyst.
The conventional method of heating the catalytic converter is to heat the catalyst by contact with high temperature exhaust gases from the engine. This heating, in conjunction with the exothermic nature of the oxidation reaction occurring at the catalyst, will bring the catalyst to light-off temperature. However, until the light-off temperature is reached, the exhaust gas passes through the catalyst relatively unchanged. In addition, the composition of the engine exhaust changes as the engine heats from the cold start temperature, and the catalyst is designed to work best with the composition of the exhaust stream present at the normal elevated engine operating temperature.
There have been several attempts to shorten or avoid the time between cold start and light-off of the catalytic converter. Current techniques employ one or more of the following methods: electrical heating of the exhaust gases and/or of the catalytic converter itself; thermal insulation; multi-chambered configurations of the catalytic converter; and/or placing the catalytic converter adjacent to the engine for heating. All of these methods have drawbacks and limitations.
Placing the catalytic converter almost immediately adjacent to the engine is not feasible because of the tendency to overheat the catalyst with resulting accelerated degradation of the catalyst due to excessive heat. Thermal insulation is also not an acceptable option because of the same problems, especially during operation under maximum operating temperature ranges.
Electrical heating of catalytic converters ("EHC") has been a popular proposed method of attempting to preheat the catalyst monoliths. Limitations on the equipment and process, however, affect the utility of this method. The primary limitation on electrical preheating is the electrical energy required by the heater. The typical car battery is not a practical power source to supply the electrical power because the electrical load on the vehicle battery during the period required may exceed the rated battery output. In any event, the load placed on a typical 12 volt vehicle battery will shorten the lifetime of the battery. Also, there is a measurable delay between the time the operator of the vehicle places the ignition switch in the "on" position and the time the heater brings the catalyst to light-off temperature.
Typically, in the interval between start up and light-off, the exhaust stream is oxygen deficient. Because the catalyst requires oxygen to complete the catalytic reaction, supplemental air must be blown over the catalyst. Even when using a secondary air flow to overcome oxygen deficiency, the secondary air flow must be closely controlled to avoid an excess of oxygen, in which case the catalytic converter is less effective in reducing NO.sub.x. However, it should be noted that NO.sub.x contributes a very small portion of unacceptable emissions when an engine is cold, most of the emissions that must be dealt with comprise HC and CO and the like.
An alternative to battery powered electrical heating has been to decrease the strain on the power supply by supplying the power directly from an alternator rather than directly from the vehicle battery. An alternator powered, electrically heated catalyst ("APEHC") still requires a 5 to 10% increase in battery capacity to cope with the EHC start-up scenario. Even with the APEHC system, there still is a concern with respect to battery capacity because electric heating is needed for an extended period of time, i.e., more than 25-30 seconds. In addition, the maximum alternator power output required in the APEHC system requires a complicated switching mechanism and an altered alternator speed between 3,000 and 4,500 rpm during the heat up time period, and the alternator must be oversized.
The multi-chamber configurations of catalytic converters generally conform to one of two theories. In one multi-chamber configuration, a small portion of catalyst known as a "starter catalyst" is positioned upstream from the primary catalyst. This "starter catalyst" is generally closer to the exhaust manifold. This location, in conjunction with a smaller thermal mass associated with its smaller size, causes the catalyst to heat much more quickly than a single catalyst. This configuration, however, is generally unacceptable because the starter catalyst in the exhaust stream creates a higher back pressure which reduces the overall engine efficiency and robs the engine of power output.
Another method of providing multiple chambers in the exhaust flow includes a first catalyst having low temperature characteristics used only during cold start conditions, and, after the catalyst temperature ranges rise to a selected elevated level, the exhaust gas flow is switched to pass through the conventional catalytic converter configuration. A variation of this approach is to run all cold start emissions through a separate absorber (such as zeolite or a metal sieve-type substance) wherein unacceptable emissions are captured and later released back into the exhaust stream. This method, however, is impractical because of the complicated switching mechanism used to divert flow to the absorber, the size and space requirements of the absorber, and the impracticality of releasing the unacceptable emissions from the absorber back into the exhaust stream.
Finally, one method runs the engine excessively rich in the cold start condition and ignites the resulting super-rich mixture to directly heat the catalyst. This approach has proved wholly unreliable and has other serious drawbacks, including reduced engine and catalyst life.
To date, there has not been a catalytic converter heating system which gives almost instantaneous heating of the catalytic converter without the inherent drawbacks stated above.
Thus, there remains a need for an improved catalytic converter system that reduces ineffective catalytic action immediately after cold start-up of an engine. Such a system must be simple and must not reduce the rated lifetime of the engine, the catalytic converter, or the battery components of the vehicle.